The Dark Ages were a period of time in which learning was slowed down due to the lack of learning materials (like books). During this time, people were usually taught by monks in monasteries or in churches. The Dark Ages lasted from the 5th to the 14th centuries.
Feudalism was a system of government in which the king, who owned the entire country, would give lands to some landowners in order to better govern the kingdom. These landowners were given the lands for their loyalty and for taxes. The king was helped by the dukes when he was being attacked or wanted to attack a country, and the dukes also paid the king a portion of their income. The duke could split his land and give it to counts, who could then give it to other people, and this goes on until the peasants, who don't own any land but worked on it.
The Crusades were a period of time in which many kings, dukes, counts, etc. went to war against the Muslims, especially to recapture Jerusalem. Some Crusades were successful, and some were not. The first and sixth Crusades were the only ones that actually accomplished the capturing of Jerusalem. The worst Crusade was the Fourth Crusade, in which the Crusaders, instead of fighting the Muslims, invaded Byzantium, their fellow Christians, for power and money.
The High Middle Ages was a period of time lasting from 1000-1250 AD. During this time were the hundred-years war, the signing of the Magna Carta, and other important events.